The present invention relates to an inverter device and, more particularly, to a voltage inverter device which has a simple, highly reliable and high-performance control circuit.
Generally, except for special cases, a voltage inverter preferably has an output voltage of a sinusoidal waveform. Various types of waveform converting means have been proposed in order to obtain a sinusoidal output from a DC voltage source. For example, an inverter is known which generates a sinusoidal pulsewidth modulated signal.
FIG. 1A shows a basic signal for obtaining a timing signal for turning on/off a main circuit switch of an inverter of this type. A reference voltage signal S.sub.1 of a sinusoidal waveform is compared with a carrier signal T of a triangular waveform to obtain timing or trigger signals (FIG. 1B and 1C) for triggering positive and negative switches, respectively, of a main circuit switch. The ON/OFF state of the main circuit switch is controlled by these signals, so that a pulsewidth modulated output voltage signal which is shown in FIG. 1D is obtained. The modulated voltage signal thus obtained may have a sinusoidal waveform indicated by a dotted line in FIG. 1D. In this manner, the sinusoidal voltage signal S.sub.2 corresponding to the reference voltage signal S.sub.1 is obtained.
In conventional sinusoidal modulation methods, a sinusoidal voltage signal is generally compared with a triangular carrier signal. However, this requires a multiplier to obtain a sinusoidal reference voltage signal by multiplying a unit sinusoidal wave by a value corresponding to a predetermined voltage. Furthermore, when the frequency of the triangular carrier wave is kept constant, a ratio of the carrier frequency to the output frequency decreases with an increase in the output frequency. As a result, a beat frequency significantly affects the output frequency. In order to suppress the beat frequency and to obtain a symmetrical output waveform, it is necessary to synchronize the output frequency and the carrier frequency. In order to obtain a symmetrical output waveform over a wide frequency range, it is still necessary to synchronize the output frequency and the carrier frequency. With a conventional inverter adopting such a conventional sinusoidal modulation method as described above, the configuration of the control circuit is complicated. In addition, in order to obtain the timing signal which turns the main circuit switch on/off, the conventional inverter requires an analog circuit comprising an operational amplifier circuit and the like for comparing the triangular wave and the sinusoidal wave. The conventional inverter is therefore likely to be adversely affected by a noise signal and so on. As a result, it has been difficult to manufacture an inverter having high performance and high reliability.